This invention relates to a semiconductor integrated circuit device. More particularly, it relates to a technique which is effective when applied to, for example, a microcomputer of single chip wherein a ROM (read only memory) to store a program for information processing is constructed of an EPROM.
In a microcomputer of single chip, predetermined information processing is carried out in accordance with a program written in a built-in ROM. The Applicant of the present application has formerly developed a single-chip microcomputer which employs an EPROM (electrically programmable read only memory) as the built-in ROM. By employing the EPROM in this manner, a single-chip microcomputer having an information processing function which a user desires can be promptly offered, and the mass-producibility of the microcomputers can be enhanced. In contrast, when a mask type ROM is employed as the built-in ROM, time is expended on the manufacture of various masks for writing the program thereof and the manufacture of an integrated circuit with the masks. The single-chip microcomputer with the built-in EPROM is described in, for example, "USERS MANUAL OF ZTAT MICROCOMPUTER HD63701X0" issued in Oct. 1985 by Hitachi, Ltd., pp. 40-42.
The EPROM built in the single-chip microcomputer as stated above is equivalently handled just as the mask type ROM, so that the erasing operation thereof is made impossible. As regards a semiconductor integrated circuit device, when a circuit is finished up on a semiconductor wafer, its function tests including write/read test are conducted by probing, and only nondefective units are subjected to the subsequent step of assemblage. At this step of assemblage, however, any defect might arise. Nevertheless, after the completion of the product, the EPROM is not erasable as described above, so that a write test cannot be conducted for the EPROM. Therefore, defective units might be shipped, and a problem is left in reliability.